This invention relates to transmissions for coupling an engine to the wheels of a vehicle and more particularly transmissions which require a large number of drive ranges to allow relatively constant and efficient engine speed over a broad range of vehicle speeds. Additionally, the invention relates to a transmission utilized in vehicles, particularly agricultural tractors, which utilize front and rear wheel drives.
Agricultural tractors have particular requirements for transmission of power to the vehicle wheels. Operation of the tractor is usually at a steady speed for long periods of time interrupted by brief periods of varying speed while the direction of the tractor and the particular work implement is changed. During the steady speed, it is desirable to maintain engine speed at its most efficient operating condition. Varying ground conditions encountered in the average agricultural application require a certain degree of flexibility in the transmission of torque during operation. This flexibility, which is deficient in present transmissions, requires a transmission which may be shifted to provide small increments of drive ratio change generally less than one mile per hour in order to achieve efficiency.
Tractors, and particularly agricultural tractors, usually incorporate a "four-wheel drive" arrangement which, in addition to a conventional transmission with an output shaft having a rearwardly facing drive coupling, generally include a transfer case. The transfer case is positioned in line and interconnected with the rearwardly facing output coupling and through appropriate gearing provides a forwardly facing drive coupling below the transmission. The associated drive shaft for transmitting power to the front wheels is affixed to the forwardly facing drive coupling and generally runs below the transmission. Such an arrangement, conventional in tractors, is expensive, complex, and introduces additional friction losses and necessitates a higher profile for the drive train with the inherent clearance problem of running a drive shaft under the transmission. Additionally, the forwardly running drive shaft is unnecessarily long, having to run from the front axle to a point to the rear of the transmission to couple with the transfer case.
In addition to the requirements for a flexible transmission and provision of means for delivering power forwardly and rearwardly, as described above, agricultural tractors have a specific requirement for a power take-off shaft to operate auxiliary equipment associated with the implement being utilized in the particular application. Generally, such a power take-off shaft in agricultural equipment is located at the rearward end of the tractor in order to properly interconnect with the associated implement.
Various agricultural tractors are on the market today which approach the conditions set forth above. However, the shift speeds while the tractor is under way generally vary from about 20% difference in speed up to about 25% difference in speed before a shift is required. In these cases, the power shifts available while the vehicle is in motion have been limited to three or less, with up to three additional manual shift gear ratios available while the tractor is stopped by direct engagement without an associated clutching unit. These transmissions currently utilized by the agricultural industry generally include planetary reduction arrangements with associated drive engagement means, and a transfer case which adds to the cost and weight of the vehicle, not to mention the inherent loss of efficiency in the rather complex arrangement.